Engine bleed air is the term used to refer to a portion of the air that has passed through all or part of an engine and is bled off for other purposes, such as to power a pneumatic system. Typically, engine bleed air is compressed air from a jet engine, such as a jet engine for an aircraft. Engine bleed air from a jet engine has a high temperature and a high pressure, so it is a potent source of energy, as well as being reliably available.
An aircraft pneumatic system uses engine bleed air as its primary source of power and serves several functions, including supplying power to (1) engine start systems (i.e., the bleed air from one running engine can be used to start another engine), (2) cabin air conditioning and pressurization systems, (3) anti-ice systems for the leading edges of the wings and the engine inlet cowls, (4) a pressurized water supply (which includes, e.g., lavatory water), (5) a pressurized hydraulic system and (6) an inert gas system, in one well known application. In a typical passenger jet aircraft, each engine has an engine air bleed system. Thus, for a Boeing 737, there is a first engine bleed air system for the left engine and a second engine bleed air system for the right engine. Under certain conditions, including when engine bleed air is not available, the pneumatic system is powered or pressurized by supplying air from a pneumatic ground air connection (i.e., while the aircraft is stationary on the ground) or the aircraft's on-board auxiliary power unit (APU). The multiple sources of power for the pneumatic system, including the left engine, the right engine, the APU and the pneumatic ground air connection, can be joined together at a manifold.
In an engine bleed air system, there is typically a valve or valves, such as a Pressure Regulating and Shutoff valve (PRSOV) in the 737 system, actuatable to move from a closed position (no engine bleed air) to an open position (full engine bleed air) to provide a flow of engine bleed air. For a 737, each valve is controlled by a bleed air switch on the pilot's instrument panel. Moving the switch to the ON position opens the valve, and when pressures are sufficient, a flow of engine air from the engine begins. Moving the switch to the OFF position closes the engine air bleed valve and blocks the flow of engine bleed air.
While engine bleed air is flowing (i.e., the PRSOV is open), the system is monitored for engine bleed air trip events that would cause undesired conditions in the engine bleed air system, such as if the temperature and/or pressure of the engine bleed air increase beyond predetermined limits. In response to sensing high temperature or high pressure of the bleed air, the valve is tripped to its closed position. A light on the pilot's instrument panel, “Bleed Trip Off,” which indicates that the valve has been tripped closed and the engine bleed air flow is off, is then illuminated.
Following an engine bleed air trip event, the valve remains closed until the engine bleed air system is reset. The pilot may wait for a short time to allow the high temperature and/or high pressure condition to dissipate, and then use a “Reset” function on the instrument panel to attempt to reset the system. If the reset is accepted, then the PRSOV is opened and the Bleed Trip Off lamp is extinguished. The PRSOV is of a butterfly valve design that is spring-loaded to a closed position and pneumatically operable to move towards a fully open position.
Within the 737 jet engine, the source of engine bleed air is the fifth and ninth stages of the high pressure compressor. There is a high stage regulator and a high stage control valve that control the flow of ninth stage bleed air. For the fifth stage, there is a fifth stage check valve that prevents flow of bleed air in the reverse direction back into the fifth stage. Engine bleed air passes through a precooler (an air to air heat exchanger), which allows the temperature of the engine bleed air to be lowered as necessary. At low engine speeds, it is the ninth stage engine bleed air that is supplied to the pneumatic system because the fifth stage flow is not sufficient, and the fifth stage check valve prevents reverse flow. At high engine speeds, the high stage valve closes and it is engine bleed air from the fifth stage that is supplied to the pneumatic system.
The manifold has a bleed air isolation valve operable to separate the manifold into left and right sides. The manifold also has a number of instruments, such as sensors, to determine the pressures and temperatures of various flows. The three positions of the bleed air isolation valve are as follows: (1) open: the right and left sides of the manifold are connected (i.e., engine bleed air from the right and left engines is combined); (2) auto: the aircraft switch position logic controls the valve to open and close as necessary for aircraft operations; and (3) closed: the valve is closed when it is desirable to separate the right and left sides of the manifold (i.e., separating the right engine and left engine flows from each other).
Bleed air trip off events occur most frequently because of a problem with the precooler, but other possible causes can arise too. Bleed air trip off events can occur during takeoff, top of ascent, cruise, top of descent, or other phase of flight when the pressure and/or temperature of the bleed air exceeds predetermined limits.
The engine air bleed system is complex and multi-faceted, so it is difficult to teach its features and their interaction with other systems of the aircraft. This difficulty is increased because the controls and indicators for the engine air bleed system are necessarily located in the cockpit for access by the pilot, but are out of view of aircraft service personnel seeking to learn their interaction with other components so troubleshooting and repair can be carried out more quickly and accurately. In addition, because the engine bleed air system only provides bleed air when the jet engine is running, it is at least difficult if not impossible to demonstrate full operation because of the close proximity of some of the components to the running jet engine.